Selected Quarterly Information (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Jun. 30, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Quarterly Information [Abstract]
Net interest income	$ 18,634us-gaap_InterestIncomeExpenseNet	$ 19,942us-gaap_InterestIncomeExpenseNet	$ 20,007us-gaap_InterestIncomeExpenseNet	$ 20,186us-gaap_InterestIncomeExpenseNet	$ 22,948us-gaap_InterestIncomeExpenseNet	$ 22,444us-gaap_InterestIncomeExpenseNet	$ 22,526us-gaap_InterestIncomeExpenseNet	$ 21,146us-gaap_InterestIncomeExpenseNet	$ 79,729us-gaap_InterestIncomeExpenseNet	$ 88,104us-gaap_InterestIncomeExpenseNet	$ 78,401us-gaap_InterestIncomeExpenseNet
Net income (loss) to common shareholders	1,379	28,572	(3,028)	19,266	(6,921)	29,442	18,381	(8,293)	18,630	60,167	72,006
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	$ 6,234us-gaap_ComprehensiveIncomeNetOfTax	$ 14,090us-gaap_ComprehensiveIncomeNetOfTax	$ 26,532us-gaap_ComprehensiveIncomeNetOfTax	$ 19,813us-gaap_ComprehensiveIncomeNetOfTax	$ (6,184)us-gaap_ComprehensiveIncomeNetOfTax	$ (67,215)us-gaap_ComprehensiveIncomeNetOfTax	$ 27,427us-gaap_ComprehensiveIncomeNetOfTax	$ 26,906us-gaap_ComprehensiveIncomeNetOfTax	$ 73,762us-gaap_ComprehensiveIncomeNetOfTax	$ (26,160)us-gaap_ComprehensiveIncomeNetOfTax	$ 127,772us-gaap_ComprehensiveIncomeNetOfTax
Net income (loss) per common share - basic and diluted	$ 0.03us-gaap_EarningsPerShareBasicAndDiluted	$ 0.52us-gaap_EarningsPerShareBasicAndDiluted	$ (0.06)us-gaap_EarningsPerShareBasicAndDiluted	$ 0.35us-gaap_EarningsPerShareBasicAndDiluted	$ (0.13)us-gaap_EarningsPerShareBasicAndDiluted	$ 0.54us-gaap_EarningsPerShareBasicAndDiluted	$ 0.34us-gaap_EarningsPerShareBasicAndDiluted	$ (0.15)us-gaap_EarningsPerShareBasicAndDiluted	$ 0.34us-gaap_EarningsPerShareBasicAndDiluted	$ 1.10us-gaap_EarningsPerShareBasicAndDiluted	$ 1.35us-gaap_EarningsPerShareBasicAndDiluted
Dividends declared per common share	$ 0.25	$ 0.25	$ 0.25	$ 0.27	$ 0.27	$ 0.29	$ 0.29	$ 0.25

Name:	dx_SelectedQuarterlyInformationAbstract
Namespace Prefix:	dx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Aggregate dividends declared during the period for each share of common stock outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

+ Details

Name:	us-gaap_CommonStockDividendsPerShareDeclared
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Other Comprehensive Income

-URI http://asc.fasb.org/extlink&oid=6519514

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Comprehensive Income

-URI http://asc.fasb.org/extlink&oid=35737396

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 220

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e557-108580

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Net Income

-URI http://asc.fasb.org/extlink&oid=6518256

+ Details

Name:	us-gaap_ComprehensiveIncomeNetOfTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

+ References

No definition available.

+ Details

Name:	us-gaap_EarningsPerShareBasicAndDiluted
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The net amount of operating interest income (expense).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.10)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 10

-Article 9

+ Details

Name:	us-gaap_InterestIncomeExpenseNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 45

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1377-109256

+ Details

Name:	us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration